Tuberculosis
Tuberculosis is a common, chronic, and frequently fatal infectious disease caused by various strains of mycobacteria, most commonly Mycobacterium tuberculosis. Drug-resistance and multi-drug resistance in tuberculosis is increasing, diminishing the efficacy of first- and second-line tuberculosis drugs. Drugs used for the treatment of tuberculosis involve the combination of multiple agents such as isoniazid, rifampicin, pyrazinamide, ethambutol, streptomycin, para-amino salicylic acid, ethionamide, cycloserine, capreomycin, kanamycin, ciprofloxacin, ofloxacin, thioacetazone, Rifapentine, Bedaquiline, and Rifampin. The regimen recommended by the US Public Health Service (http://www.hhs.gov/pharmacy/pp/DHHSpresent/) is a combination of isoniazid, rifampicin, and pyrazinamide for two months, followed by isoniazid and rifampicin, together, for another four months. These drugs are continued for another seven months in patients infected with HIV. For the treatment of multi-drug resistant tuberculosis, streptomycin, kanamycin, amikacin, capreomycin, ethionamide, cycloserine, ciprofloxacin, and ofloxacin are added to the combination therapies (World Health Organization, Anti-tuberculosis drug resistance in the world Third Global Report 2004). Currently, there is neither a single agent nor a combination therapy that can both treat tuberculosis and shorten the duration of treatment. All existing approaches to tuberculosis treatment involve the combination of multiple agents. No single agent exists that is effective in the clinical treatment of tuberculosis, nor is there any combination of agents that offer the possibility of a therapeutic regimen having less than a six month duration. An urgent need exists for novel and potent inhibitors of pathogenic mycobacteria.
Mycobacterium tuberculosis (Mtb) is characterized by an unusually lipid-rich cell wall of low permeability which allows the bacterium to survive in the hostile environment of the macrophage and cause infection. Mycobacterial lipids are essential for both viability and pathogenicity.
The first step of fatty-acid biosynthesis is mediated by acyl-CoA carboxylase (ACC). ACC catalyzes the carboxylation reaction of acetyl-CoA to produce malonyl-CoA, a precursor in long chain fatty acid biosynthesis. These fatty acids are essential for survival, virulence, and antibiotic resistance in Mtb. In particular, the D6 carboxyltransferase β-subunit (AccD6) has been shown to be essential to pathogenic mycobacteria, indicating that this enzyme represents an ideal target for inhibition. The AccD6 gene in M. bovis shares complete sequence identity with that of Mtb.
Most bacteria have a multi-subunit ACC composed of three functional polypeptides: BC (AccC), BCCP (AccB), and CT (AccA plus AccD) (2). For example, in Escherichia coli and Staphylococcus aureus, these Accs are composed of three independent (BC, BCCP, and CT) functional proteins (2). In yeast and mammals, these functions are carried out by a single polypeptide with distinct BC, BCCP, and CT domains (3). In comparison, the Mtb genome contains three BC α-subunits (AccA1 to Acc3) and six CT β-subunits (AccD1 to D6) (14). The high number of β-subunits is unusual as other bacteria generally only have 1-2 ACCases. The multiple β-subunits likely reflect the ability of mycobacteria to carboxylate other distinct substrates, including the short acyl CoAs used as intermediates in glycolipid biosynthesis. Therefore, the presence of multiple AccA and AccD genes contained within the Mtb genome is thought to be linked to the wide variety of lipids found in Mtb.
ACC Inhibitors
Arylphenoxypropionate derivatives are potent inhibitors of ACCs, and several arylphenoxypriopionate derivatives, including haloxyfop, are currently used in herbicides in light of their species-dependent ACC inhibition. Commercially available arylphenoxypropionate derivatives exhibit little human toxicity. Quizalofop-p-ethyl, for example, has LD50 values of 1753 to 2350 mg/kg in male mice and 1805 to 2360 mg/kg in female mice. In rabbits, it was reported that LD50 values were greater than 2,000 mg/kg. Also it was shown in a 1-year feeding study on dogs that doses of up to 10 mg/kg/day caused zero observed effects. This compound is rapidly broken down in mammals; more than 90% of a single oral dose is eliminated in urine within three days. The Carcinogenicity Peer Review Committee CPRC has classified quizalofop ethyl as a Group D carcinogen (i.e., not classifiable as to human cancer potential). To date, however, there are no bacterial ACC inhibitors in clinical use as antibiotics.